Care to do a experiment

[ChaosKnight]

i can try with larger F-ratios, but i doubt the effect is due to spherical aberration. If i understand correctly, spherical aberration is due to the spherical surface being unable to bring all the light to a focal point. But so far i don't see this effect in the drawings. Maybe this effect is so small that inaccuracies when using set square/pencil overshadow it. My pencil lead diameter is already .3mm.

[ChaosKnight]

i tried with a ray trace program (thanks for supplying the term, Kay Heem) and the results are the same. You can download the program here:

http://members.ozemail.com.au/~imesoft/raytrace.htm

[kayheem]

i tried with a ray trace program (thanks for supplying the term, Kay Heem) and the results are the same. You can download the program here:

http://members.ozemail.com.au/~imesoft/raytrace.htm

OK, assuming your theory holds, it leads on to:

1) For poor seeing, e.g. Singapore, what is the aperture for best compromise? Surely it would not be a 60mm refractor. If it is, all our aperture-fever would have been in vain

2) Would the same problem be present in refractors? Could the problem be coma instead of spherical aberration?

3) Let's get on with a real-world test! We can use the same aperture mask and position it near and far from the optical axis. We will then have the same aperture, same F-ratio but utilising different part of the mirror. Only thing is that we will have to use a premium mirror to ensure a smooth and regular figure throughout the reflecting surface.

Kay Heem

P.S. Off-axis reflectors use the outer part of a larger parabolic mirror, and the image is reputedly excellent. This does not follow the theory and ray trace diagram.

[ChaosKnight]

i tried with a lens instead of a mirror. Guess what: with a slight skew in incident rays, the refracted rays still all converge on the same point, at the same distance from the original focal point! Aperture size does not affect image quality! Yay i was right to get a refractor!

Surprisingly, by making the focal length of the primary shorter, the new focal point lies closer to the original focal point. This suggests that for refractors, a SHORTER FOCAL LENGTH PERFORMS BETTER IN TURBULENCE.

No wonder my test showed no evidence that stopping down aperture improves image quality.

I think it's possible off-axis reflectors using the outer regions of the primary will give excellent images under steady seeing. In severe turbulence, they will likely suffer more.

It's up to the reflector guys to prove/debunk this hypothesis.

[kayheem]

i tried with a lens instead of a mirror. Guess what: with a slight skew in incident rays, the refracted rays still all converge on the same point, at the same distance from the original focal point! Aperture size does not affect image quality! Yay i was right to get a refractor!

Surprisingly, by making the focal length of the primary shorter, the new focal point lies closer to the original focal point. This suggests that for refractors, a SHORTER FOCAL LENGTH PERFORMS BETTER IN TURBULENCE.

No wonder my test showed no evidence that stopping down aperture improves image quality.

I think it's possible off-axis reflectors using the outer regions of the primary will give excellent images under steady seeing. In severe turbulence, they will likely suffer more.

It's up to the reflector guys to prove/debunk this hypothesis.

OK, I tried out an example using a parabolic mirror on the ray trace program you suggested. My observations/comments:

1. Yes it is true that rays away from the optical centre is more affected than those nearer the optical axis.

2. This looks like it is due to coma.

3. The F-ratio the program generated is way to low It looked like F/1 or so. Real-life F-ratios of 8 would probably have neglible results. Is there an option for coma correction?

4. Refractors are not prone to coma.

Anyone else has an opinion on this?

Kay Heem

[ChaosKnight]

Yes the situation now seems to be the prerequisite for coma, isn't it?

As i understand it, coma is due to incorrect collimation. Turbulence in the atmosphere causes your otherwise perfect rays to hit the primary at a skewed angle. So your perfectly collimated mirror is now subjected to coma. Coma correction is not a factor here. It's not your mirror that has a problem. It's the incoming rays. And in real life, these rays are always shifting, changing their skew angle. No coma correction can solve that.

It's true that the optical systems simulated are quite fast. What i did was an f/3 or so. But i think if you do a really slow mirror, the results will be the same, just that you need to zoom in to see the same phenomenon. Slower scope handles turbulence better.

But then again, this just proves the same point: assuming same focal length, a faster telescope, compared to a slow one, is of bigger aperture. It is more affected. Slower (ie smaller aperture) performs better.

It's true that in real life systems with larger f-ratios, the effect is not as great. The incident rays don't come skewed at a very big angle either. This is probably why you only see underwater images. If you translate the simulations to real life, ie a great change in angle of incidence (3 degress or so) and a very fast optical system, you wouldn't just see underwater images. You will likely see the image jiggling in and out of your FOV.

[kayheem]

As i understand it, coma is due to incorrect collimation.

Coma, as I understand it, is a problem of a parabola. The faster the parabola, the worse it is. This has nothing to do with collimation. It exists in a perfectly collimated scope.

Let me try to recall some of your points (do correct me if I am wrong):

1. refractors are not so affected by aperture under conditions of poor seeing.

2. large aperture reflectors are more affected by poor seeing, mainly because of aperture

Just for discussion sake:
Are you suggesting that, if there are 2 reflectors of the same F-ratio, operating at the same magnification but at different apertures, the larger aperture scope will be affected more?

How about 2 scopes with the same aperture, same magnification but different F-ratio? Will the faster scope be affected more?

Would point sources (i.e. stars) and extended objects be equally affected?

What I do not understand is, why is it only the 2 of us discussing this topic when there are a lot of reflector users or ex-reflector users in Singastro?


Kay Heem

[ChaosKnight]

Let me restate more clearly our observations so far:

For refractors of same focal length, hence same magnification, but different apertures, hence different f-ratios, they are both equally affected by poor seeing.

For two reflectors having the same f-ratio and magnification but different apertures, i speculate that they are both equally affected. You have to draw the ray diagrams to find out.

For 2 scopes with same aperture but different f-ratio, i speculate the faster one will be affected. This is because the curvature of the mirror at the edge is higher.

I think it is likely that small point sources like stars will form distorted images, but the distortion is so small it is hardly noticeable. Probably will show up as twinkling as the distortion changes form.

As for your last point, judging from the frequency of your posts, don't you have a day job too?

[kayheem]

For two reflectors having the same f-ratio and magnification but different apertures, i speculate that they are both equally affected. You have to draw the ray diagrams to find out.

....As for your last point, judging from the frequency of your posts, don't you have a day job too?

Hmm...did I misread you or did you change from aperture to f-ratio being the determinant of image degradation in reflectors in poor seeing?

Kay Heem

P.S. I was very tickled by your last sentence.

[ChaosKnight]

It was always about the f-ratio, just that in our context so far, we were keeping focal length constant and varying aperture.
i know the implications of the above statement: a fast 6" newt (f/5??) will perform worse than a regular 8" f/10.